Mapping out sculpted details from a high-poly
I dare to post this answer, because the following tool is free and in my opinion everyone should know this great piece of software. Please forgive me! :-) 

After some testing the best working solution I found so far can be archived by creating a heightmap in XNormals. Therefore I hope that you are on a windows system, because XNormals is a pure windows tool. 


Export your highpoly and your lowpoly  and a cage as seperate Fbx scenes.





Import them to XNormal (just drag the highpoly and the lowpoly into the list).




for the cage, a checkbox in the lowpoly object has to be enabled and you have to select it by clicking with RMB in the sceen and select "Browse external cage file"

 


In the baking option select your texture size you want to export, your export location, and check heightmap as renderer. 





To render, press the Generate Maps button and after some calculating something like this should appear:




After adjusting your heightmap, close the window  and it will export it to your predefined export location.